1. Field of the Invention
The general inventive concept relates to a light guide plate, a backlight unit, a liquid crystal display (LCD), and a method of manufacturing the light guide plate, and more particularly, to a light guide plate capable of achieving an enhancement in light efficiency while ensuring a desired uniformity of luminance of a backlight unit used in an LCD, and a method of manufacturing the light guide plate.
2. Description of the Related Art
LCDs, which are widely used in notebook computers and monitors, require a separate illumination device because they employ elements having no function of emitting light by themselves, as compared to cathode ray tubes (CRTs). Illumination devices for LCDs are classified into a reflection type, a transmission type, and a combination type thereof in accordance with operating systems thereof. The transmission type illumination device, which is most widely used, is also called a “backlight unit” since it irradiates illumination light onto the backside of a liquid crystal panel.
Among various types of backlight units, a sidelight type is widely used. The reason why this type of backlight unit is called a “sidelight type” is that a light source for emitting illumination light is arranged at one side of a light guide plate adapted to propagate the illumination light therethrough. Such a backlight unit includes a light source, a light guide plate for directing the light emitted from the light source toward a liquid crystal panel, a prism sheet for correcting the directionality of the light going out of the light guide plate, thereby directing the outgoing light to a front side of the liquid crystal panel, and a diffusion sheet for relaxing the directionality of the light outgoing from the prism sheet.
The light guide plate generally has a tapered plate structure having a thickness gradually reduced as it extends away from the light source. The light guide plate has an incidence face provided by one of the minor faces thereof facing the light source, and an outgoing face provided by one of the major faces thereof and adapted to allow light propagated through the light guide plate to exit the light guide plate. The other major face is referred to as an “opposite face or backside face,” and is opposite to the outgoing face.
The illumination light exiting the outgoing face of the light guide plate exhibits a non-uniform distribution of light amount over the outgoing face because the amount of light thereof in the vicinity of the light source is relatively small. As a result, the outgoing face of the light guide plate has a region exhibiting a relatively low luminance as compared to other regions. This is because the light exiting the outgoing face of the light guide plate has a large amount of components having an exiting angle larger than a critical total-reflection angle at a region of the outgoing face in the vicinity of the light source, so that these light components are totally reflected, thus being re-propagated into the light guide plate without exiting the outgoing face.
In order to eliminate such a low luminance region, thereby ensuring a desired luminance uniformity, various methods have been proposed. For example, Japanese Patent Laid-open Publication No. 11-119219 discloses a method in which a light scattering pattern is provided at a low luminance region of an outgoing face by locally roughening the low luminance region. Also, Japanese Patent Laid-open Publication No. 2001-083507 discloses a method in which the entire portion of an outgoing face is roughened such that the roughness at a low luminance region of the outgoing face is larger than that of other regions. For the manufacture of such a roughened light guide plate, an injection mold partially processed by a blasting or etching method may be used.
In accordance with the above mentioned conventional methods, the mean inclination of a light guide plate at an outgoing face or backside face thereof is increased to cause illumination light exiting the outgoing face to have an increased amount of components having an exiting angle smaller than a critical total-reflection angle, thereby achieving an increase in the exiting rate of the illumination light.
In the case of a light guide plate roughened at an outgoing face or backside face thereof, as described above, however, scattering of the exiting light may occur. As a result, there may be an increase in the loss of light caused by haze. Furthermore, where such a roughened face is formed using an etching process, its shape is irregular, so that it is impossible to implement a desired mean inclination at all regions of the roughened face. For this reason, there is a problem in that luminance defects may be easily generated. In addition, there is a difficulty in accurately reproducing the roughened face.